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An interpretation of initiation pressure hysteresis phenomena for fixed‐point pressure calibration is presented according to the theory of nucleation in condensed systems and is substantiated for the bismuth I–II point by hydrostatic pressure experiments with a manganin pressure gauge. Kinetic initiation and completion of the bismuth I–II transformation were observed at constant pressure and temperature, and several different isobaric initiation pressures were measured at the same temperature. An asymmetrical relationship of the equilibrium pressure between the initiation pressures of the forward and reverse transformations was measured, and it was demonstrated that the high‐pressure side of the strain hysteresis of initiation is within 0.17 kbar of the equilibrium pressure. The microstructure and thickness of the sample influenced the initiation pressures and the region of indifference, but the center of the region appeared reproducible. A region of indifference of 0.03 kbar was measured. The origin of the real strain hysteresis of initiation is identified for polymorphic transformations and initiation pressures are discussed in terms of the strain energy and chemical kinetics of nucleation. It is shown that other interpretations of hysteresis phenomena are not satisfactory. It is reasoned that the equilibrium pressure rather than a higher pressure value is a more accurate calibration pressure for the initiation of the bismuth I–II transformation in solid‐media apparatus.